Gas turbine engines have turbine rotor blades which rotate relative to a surrounding casing. To reduce heat generation, protect the blade and to form a seal between the blade and the casing, an abrasive coating may be attached to the blade tip. For example, FIG. 1a shows a smooth tipped turbine blade 31 with an abrasive coating 33, and FIG. 1b a cross section through the blade and coating. The abrasive coating comprises hard particles 35 embedded in a retaining matrix 37. When the blade is installed in a turbine and rotates, the hard particles abrade the softer material of the surrounding casing such that the blade forms a groove in the casing surface, providing a tight clearance and reducing friction between the blade and surrounding casing.
When attaching the abrasive coating, the hard particles may be tacked to the blade tip to hold them in place before the matrix is applied. Near to the edge of the blade tip, these tacked hard particles may drop off. This is particularly problematic when an abrasive coating is applied to a narrow section. For example, FIG. 2a shows a squealer tipped turbine blade 31 with an abrasive coating 33, and FIG. 2b shows a cross section through the blade and coating. The abrasive coating, containing the hard particles 35 and the retaining matrix 37, is attached to the narrow projecting lips 38 of the squealer tip. Due to their location close to the edges of the lips, hard particles may fall off. This may result in the abrasive coating having a reduced number of hard particles, decreasing the effectiveness of the coating.
A further problem arises if hard particles located at an edge encourage matrix material to be laid down overhanging the edge. Such overhangs can increase aerodynamic losses and may interfere with blade film cooling in the adjacent aerofoil surface.
Moreover, the abrasive coating on both the smooth and the squealer tipped blades is generally attached to a smooth surface. At elevated temperatures under near plastic conditions, the strength of the coating or the strength of the attachment between the coating and smooth surface may be insufficient to prevent the coating from being smeared off.